1. Field of the Invention
This invention relates to scanning electro-optical systems. More specifically, the present invention relates to methods and apparatus for the control of the footprint of a scanning beam as the beam scan angle increases.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those of ordinary skill in the art having access to the teachings provided herein will recognize additional modifications, applications and embodiments within the scope thereof.
2. Description of the Related Art
Scanning radiometers and other electro-optical detection instruments typically utilize a detector (or set of detectors) which are adapted to receive radiant energy directly or by reflection from a scan mirror. In angular scanning systems, the scanning mechanism traverses a scan angle on either side of a line in the middle of the angular scanning range. This line is known in the art as the nadir.
At a given scan angle, the field of view of the detector projects a footprint onto the scanned surface which is the instantaneous size of the projected detector image on the surface. In most applications, the scanned surface is either planar or curved in such a way that as the scan angle increases, the area of the footprint increases from a minimum at the nadir to a maximum at a maximum scan angle.
The growth of the footprint as a function of the scan angle inhibits the ability of airborne and satellite borne scanning radiometers to achieve a constant ground spatial resolution at large scan angles off of the nadir. This presents a particular problem in applications where there is a need to examine a fixed area on the scanned surface with a field of view of constant spatial resolution irrespective of scan angle.
One prior approach to the constant footprint problem includes limiting the scan angle to provide a substantially constant field of view close to the nadir. This approach, however, requires more time to cover the same surface area. Accordingly, its slow speed may force other compromises on the system making it undesirable for many applications.
Another approach is embodied in the Defense Meteorological Satellite Program's operational linescan radiometer. The operational linescan radiometer attempted to achieve a constant ground footprint in the scan (and track) direction by changing the size of the instantaneous field of view as a function of scan angle. The operational linescan radiometer used a combination of techniques including switching to smaller detector sizes during the scan and use of rectangular detectors whose instantaneous field of view rotated with the scan. However, the resulting performance was found to be less than optimum. That is, the degree of constant spatial resolution achieved was limited and the radiometric resolution was compromised.
Thus, there is a need in the art for an improved method and apparatus for use in scanning radiometers which would be effective to maintain a constant footprint (i.e., an area of constant spatial resolution) at a target surface at large scan angles off nadir, without limiting the instantaneous field of view of the detector nor the radiometric resolution thereof.